Genetic and enzymatic analysis of 17 L-arabinose nonutilizing mutants of Escherichia coli B/r, together with eight previously analyzed mutants of the B gene, show no relationship between the location of a mutant site in the B gene and its effect on the level of inducib1e L-arabinose isomerase, the product of the adjacent structural A gene. The results of reciprocal three-factor crosses were used to estab1ish the linear genetic order of most arabinose negative mutants. Other mutants could not be ordered by this method because of high negative interference or close linkage. Recombination distances were used to establish the probable order of most of these mutants. (auth)